Shear Zones

Abstract

A subject that has fascinated many geologists over the last decades is treated in this Chapter. Concentration of deformation along shear zones, producing mylonites or brittle fault rocks is discussed as related to depth and therefore metamorphic environment. The different fault rocks are treated from low to high grade or from fast to slow strain rates. The first part of the chapter deals with a description of brittle fault rocks that may be cohesive or incohesive, and of pseudotachylytes that form by seismic events.

Mylonites are produced predominantly by ductile deformation and usually show significant recrystallisation of the matrix. Characteristic fabric elements are porphyroclasts in a finer grained matrix.. Attention is given to the dynamics of mylonite development and to the influence of different metamorphic conditions. Many shear zones show evidence of repeated activity under different metamorphic conditions or strain rates, producing e.g. narrow cataclasites cutting mylonite zones.

The determination of shear sense in shear zones is of crucial importance for the reconstruction of the tectonic evolution of e.g. colliding continents, crustal strike-slip movements or escape tectonics. A number of well established shear sense indicators is presented and discussed. These include: displacement of markers, foliation curvature, shear band cleavage including C/S fabrics, mantled porphyroclasts, mineral fish, quarter structures and lattice preferred orientation. Other more problematic shear sense indicators are also evaluated. Special attention is dedicated to a new topic called flanking structures, in which crosscutting elements such as faults or veins transect the mylonitic foliation at an angle. The last section is on shear sense indicators in the brittle regime.